This invention relates to a method of producing an information recording medium for use as a recording medium for an information processing apparatus and to a method of producing a substrate for the information recording medium. This invention also relates to the information recording medium for use as the recording medium and to the substrate for the information recording medium.
A magnetic disk is known as one of information recording media for use as a recording medium for an information processing apparatus. The magnetic disk comprises a substrate and a thin film such as a magnetic layer formed thereon. With a recent demand for an increase in recording density, a flying height which is a distance between the magnetic disk and a magnetic head has a very small value, for example, equal to 30 nm or less. As a consequence, it is desired to provide a substrate having a surface high in flatness and smoothness. The substrate having a surface high in flatness and smoothness can be obtained by appropriately selecting a polishing pad used in a polishing step or by reducing a particle size of abrasive grains used in the polishing step.
In addition to the improvement in flatness and smoothness of the surface of the substrate, the magnetic head is improved from a thin film head to a magnetoresistive head (MR head) and a giant magnetoresistive head (GMR head) utilizing an anisotropic magnetoresistance in order to meet the increase in recording density.
As described above, the high flatness of the surface of the magnetic disk is essential and indispensable in order to achieve a low flying height required to increase the recording density. In case where the MR head is used, the surface of the magnetic disk is required to be high in flatness and smoothness in view of thermal asperity also. The thermal asperity is a phenomenon such that, if a protrusion is present on the surface of the magnetic disk, the MR head is affected by the protrusion to generate heat and, as a consequence, the MR head becomes unstable in resistance value to cause malfunction in electromagnetic conversion.
Thus, in order to reduce the flying height of the magnetic head and to prevent occurrence of the thermal asperity, the demand for the high flatness and the high smoothness of the surface of the magnetic disk is more and more increasing day by day.
However, at the present stage, the increase in recording density of the magnetic disk can not be achieved only by polishing the surface of the substrate with high precision. Even if high-precision polishing is performed, protrusions may thereafter be formed on the substrate due to presence of foreign matters. In this event, the high flatness and the high smoothness of the magnetic disk can not be achieved. In fact, the removal of the protrusions due to presence of the foreign matters is already performed. However, the protrusions on the substrate, which are very small and need not be removed in the past, cause a serious problem at a present level of the increase in recording density.
If the thin film such as the magnetic layer is deposited on the substrate with the protrusions of the type attached to the surface of the substrate, protrusions are formed on the surface of the magnetic disk to become a factor inhibiting the reduction in flying height of the magnetic head and the prevention of occurrence of the thermal asperity (prevention of occurrence of a recording or a reproducing error).
Likewise, if the thin film such as a recording layer is deposited on the substrate with the protrusions of the type attached to the surface of the substrate, protrusions are formed on the surface of the information recording medium to become a factor causing a defect such as the recording or the reproducing error.
It is an object of this invention to provide a method of producing a substrate for an information recording medium, which is capable of suppressing and preventing a protrusion from being formed on a substrate to act as a factor inhibiting a reduction in flying height of a magnetic head and a prevention of occurrence of thermal asperity and/or as a factor causing a defect in an information recording medium.
It is another object of this invention to provide a method of producing an information recording medium using the above-mentioned substrate.
It is still another object of this invention to provide a substrate for an information recording medium which is capable of suppressing the influence of protrusions acting as a factor inhibiting a reduction in flying height of a magnetic disk and a prevention of occurrence of thermal asperity and/or as a factor causing a defect in the information recording medium.
It is a further object of this invention to provide an information recording medium using the above-mentioned substrate.
The present inventors found out that, even if a substrate is washed and dried in a washing/drying step, some protrusions may be left without being diminished and that, if a thin film such as a magnetic layer is deposited on the substrate with the protrusions attached to the surface of the substrate, protrusions are formed on the surface of a magnetic disk to become a factor inhibiting a reduction in flying height of a magnetic head and a prevention of occurrence of thermal asperity.
It has also been found out that the protrusions contain at least one of C, O, Al, Si, Fe, Cu, Zn, and Zr present in a cleaning liquid or an atmosphere.
The present inventors have investigated the reason why the protrusions containing the above-mentioned element or elements are attached to the substrate. As a consequence, it has been found out that, if the surface of the substrate is washed or rinsed with water and thereafter dried, the protrusions are left on the substrate at local spots where the water is deposited and then dried. It has also been found out that some of the protrusions containing the above-mentioned element or elements can not be removed by various washing techniques and can only be removed by polishing the substrate again.
As the water used in the washing/drying step, use is generally made of filtered water, DI water (deionized water), and the like. The washing/drying step is carried out in a clean atmosphere within a clean room or in an atmospheric air. The water or the atmosphere mentioned above contains C, Al, Si, Fe, Cu, Zn, Zr, and the like. It has been found out that the water containing such element or elements, in combination with the surface condition (hydrophobic nature) of the substrate, is left in particular regions to form the protrusions. Even if the water containing such element or elements has a very small amount on the order of ppb, the protrusions are formed on the surface of the substrate.
It has been found out that the product defects due to presence of the protrusions containing at least one of C, O, Al, Si, Fe, Cu, Zn, and Zr can be avoided by determining and controlling the content of such element or elements contained in the water used in the washing/drying step and causing the protrusions so that the product defects are avoided. This leads to the completion of this invention.
Furthermore, investigation has been made about how the protrusions are formed. As a result, it has been found out that the size (height) of the protrusions is closely related to the wettability of the surface of the substrate before washing. Specifically, if the surface of the substrate before washing is inferior in wettability (i.e., hydrophobic), water droplets of the cleaning liquid locally stay on the surface of the substrate and are dried. In this event, those elements contained in the water or the atmosphere concentrate to the particular regions to form the protrusions. On the other hand, if the surface of the substrate before washing is superior in wettability (i.e., hydrophilic), the water droplets of the cleaning liquid spread over the surface of the substrate and are dried. In this event, those elements contained in the water or the atmosphere do not concentrate to the particular regions but are dispersed throughout the surface of the substrate. Therefore, no protrusions are formed or, even if the protrusions are formed, the height of the protrusions is too small to cause occurrence of head crash or the thermal asperity.
As an index representative of the surface condition (wettability) of the substrate, use is generally made of a contact angle of water. The contact angle of water is an angle formed between a surface of an object and a water droplet sitting on the surface. It has been found out that the protrusions tend to be produced in case where the contact angle of water on the surface of the substrate before the washing/drying step exceeds 20xc2x0.
It has been found out that, in a production process of a substrate for an information recording medium, product defects can be avoided by the use of a correlation between the contact angle of water and the size (height) of the protrusions. Specifically, measurement is made of the contact angle of water on the surface of the substrate before the washing/drying step. Measurement is also made of the size (height) of the protrusions attached to the surface of the substrate after the washing/drying step and containing at least one of C, O, Al, Si, Fe, Cu, Zn, and Zr as a main component. Then, the correlation between the contact angle of water and the size (height) of the protrusions is obtained. Based on the correlation, the contact angle of water on the surface of the substrate before the washing/drying step is controlled so that the protrusions will have such a size (height) that no product defects are caused.
The wettability of the surface of the substrate is related to the surface condition of the surface of the substrate and is generally represented by the contact angle of water. The contact angle of water is typically measured by a method called a sessile drop method.
The surface condition (the wettability or the contact angle of water) of the surface of the substrate depends upon treatment techniques (such as in the washing/drying step) upon production of the substrate, the environment upon production and inspection of the substrate, the environment during storage of the substrate, such as a packaging condition of the substrate and the type of a bag packaging the substrate (the amount of a gas released from the bag, the number of particles produced from the bag, and so on), the storage period of the substrate, the surface roughness of the substrate, the material of the substrate, the surface treatment (for example, hydrophilic treatment) of the substrate, and the like. A combination of those factors determines the wettability of the surface of the substrate. It has been found out that not only the washing/drying step but also those factors must be controlled to control the contact angle of water on the surface of the substrate before the washing/drying step. Based on these findings, this invention has been made.
Thus, it has been found out that the product defects can be avoided by defining the height of the protrusions formed on the surface of the substrate and containing at least one of C, O, Al, Si, Fe, Cu, Zn, and Zr to a predetermined value or less. This leads to the completion of this invention.
This invention includes the following structures.
Structure 1-1
A method of producing a substrate for an information recording medium, the method including a washing/drying step of washing and drying the substrate subjected to precision polishing, the method comprising:
obtaining a correlation between a contact angle of water on a surface of the substrate before the washing/drying step and a size (height) of protrusions attached to the surface of the substrate after the washing/drying step; and
controlling (setting) the contact angle of water on the surface of the substrate before the washing/drying step so that the protrusions will have such a size (height) that no hit is caused when at least a recording layer is formed on the substrate to produce the information recording medium and a slider provided with a recording device and/or a reproducing device is made to run on a surface of the information recording medium and/or that no error is caused upon carrying out a recording operation and/or a reproducing operation.
Structure 1-2
A method of producing a substrate for an information recording medium, the method including a washing/drying step of washing and drying the substrate subjected to precision polishing, wherein:
a correlation is established between a contact angle of water on a surface of the substrate before the washing/drying step and a glide test fail rate in a glide test carried out by forming at least a recording layer on the surface of the substrate to produce the information recording medium and by causing a slider provided with a recording device and/or a reproducing device to run on a surface of the information recording medium;
an additional correlation being also established between the contact angle of water on the surface of said substrate before said washing/drying step and an error rate where recording and/or reproducing are carried out for the information recording medium produced by forming at least said recording layer on the surface of said substrate;
the contact angle of water on the surface of said substrate having a value such that the glide test does not result in failure and that a recording error and/or a reproducing error are not caused when the recording and/or the reproducing are carried out for the information recording medium produced by forming at least said recording layer on the surface of said substrate.
Structure 1-3
A method as described in the structure 1-1 or 1-2, wherein the contact angle of water on the surface of the substrate before the washing/drying step is equal to 20xc2x0 or less.
Structure 1-4
A method of producing a substrate for an information recording medium, the method including the step of packaging the substrate obtained through a washing/drying step, wherein:
the contact angle of water on a surface of the substrate is kept at 20xc2x0 or less at least immediately before pre-washing and drying the substrate prior to deposition of a film on the substrate after it is unpacked.
Structure 1-5
A method as described in the structure 1-4, wherein:
the contact angle of water on a surface of the substrate at is kept at 20xc2x0 or less for a period immediately after the washing/drying step and immediately before a pre-washing and drying step performed before a film is deposited on the substrate which is unpacked.
Structure 1-6
A method as described in any one of the structures 1-1 through 1-5, wherein the substrate is a glass substrate.
Structure 1-7
A method as described in any one of the structures 1-1 through 1-6, wherein the substrate is a substrate for a magnetic disk.
Structure 1-8
A method of producing an information recording medium, the method including the steps of preparing a substrate subjected to precision polishing, washing and drying the substrate before depositing a film thereon, and forming at least a recording layer on the substrate after the washing and drying step, the method comprising:
obtaining a correlation between a contact angle of water on a surface of the substrate prior to washing in the washing and drying step and a size (height) of protrusions attached to the surface of the substrate after the washing and drying step; and
controlling (setting) the contact angle of water on the surface of the substrate prior to washing in the washing and drying step so that the protrusions will have such a size (height) that no hit is caused when at least a recording layer is formed on the substrate to produce the information recording medium and a slider having a recording device and/or a reproducing device is made to run on a surface of the information recording medium and/or that no error is caused upon carrying out a recording operation and/or a reproducing operation.
Structure 1-9
A method of producing an information recording medium, the method including the steps of preparing a substrate subjected to precision polishing, washing and drying the substrate before depositing a film thereon, and forming at least a recording layer on the substrate after the washing and drying step, wherein:
a correlation is established between a contact angle of water on a surface of the substrate before the washing and drying step and a glide test fail rate in a glide test carried out by forming at least the recording layer on the surface of the substrate to produce the information recording medium and by causing a slider provided with a recording device and/or a reproducing device to run on a surface of the information recording medium;
an additional correlation being also established between the contact angle of water on the surface of said substrate before said washing and drying step and an error rate where recording and/or reproducing are carried out for the information recording medium produced by forming at least said recording layer on the surface of said substrate;
the contact angle of water on the surface of said substrate having a value such that the glide test does not result in failure and that a recording error and/or a reproducing error are not caused when the recording and/or the reproducing are carried out for the information recording medium produced by forming at least said recording layer on the surface of said substrate.
Structure 1-10
A method as described in the structure 1-8 or 1-9, wherein the contact angle of water on the surface of the substrate prior to washing in the washing and drying step is equal to 20xc2x0 or less.
Structure 1-11
A method as described in any one of the structures 1-8 through 1-10, wherein the washing and drying step uses spin dry.
Structure 1-1 2
A method as described in any one of the structures 1-8 through 1-11, wherein the substrate is a glass substrate.
Structure 1-13
A method as described in any one of the structures 1-8 through 1-11 wherein the substrate is a substrate for a magnetic disk.
According to the structure 1-1, the contact angle of water on the surface of the substrate before the washing/drying step (particularly before drying) as a factor causing protrusions containing at least one of C, O, Al, Si, Fe, Cu, Zn, and Zr is set (determined, preferably, controlled) so that product defects can be avoided. Thus, it is possible to avoid product defects.
Specifically, the contact angle of water on the surface of the substrate before the washing/drying step is set that, when at least the recording layer is formed on the substrate to produce the information recording medium, protrusions will have a size (height) too small to hit the slider provided with the recording and/or the reproducing device and/or that no error is caused upon carrying out the recording operation and/or the reproducing operation. Therefore, it is possible to prevent occurrence of head crash and thermal asperity and to eliminate negative factors causing a defect such as an error in the recording operation and/or the reproducing operation of the information recording medium.
In this invention, the washing/drying step involves a washing operation immediately after precision polishing (final polishing), a washing operation after chemical reinforcement, a washing operation before shipment, a washing operation immediately before deposition, or any other washing operation carried out after the precision polishing and before the deposition. Once the protrusions including at least one of C, O, Al, Si, Fe, Cu, Zn and Zr are formed in any one of the washing/drying steps corresponding to the above-mentioned washing operations, such protrusions may not be removed by the subsequent washing step or steps. Therefore, it is preferable that the contact angle of water on the surface of the substrate before each of these washing/drying steps is determined and controlled so as to avoid product defects.
In the structure 1-1, it is sufficient that the contact angle has a value such that the product defects are avoided. Therefore, the contact angle need not be so small as to require an excessively high cost. In other words, the contact angle can be determined taking the cost into consideration.
The size (height) of the protrusions may be a value obtained by directly measuring only the protrusions containing at least one of C, O, Al, Si, Fe, Cr, Zn, and Zr or may be a value obtained by measuring the surface roughness of the substrate before and after the washing/drying step.
With the structure 1-2, the contact angle of water on the surface of the substrate has a value such that the glide test does not result in failure and that a recording error and/or a reproducing error are not caused. Thus, it is possible to produce the substrate for the information recording medium capable of avoiding the defect and the error mentioned above.
In the structure 1-3, the contact angle of water on the surface of the substrate for the information recording medium before the washing/drying step (particularly before drying) of the substrate is controlled to be equal to 20xc2x0 or less so that the height of the protrusions formed on the substrate and containing at least one of C, O, Al, Si, Fe, Cu, Zn, and Zr is reduced so as to avoid the product defects when the information recording medium is produced.
As the contact angle of water on the surface of the substrate for the information recording medium before the washing/drying step of the substrate is smaller, the height of the protrusions containing at least one of C, O, Al, Si, Fe, Cu, Zn, and Zr is reduced. Specifically, the contact angle of water on the surface of the substrate before the washing/drying step is preferably equal to 5xc2x0 or less, more preferably, 10xc2x0 or less.
With the structure 1-4, in the method of producing the substrate for an information recording medium including the step of packaging the substrate obtained after the washing/drying step, the contact angle of water on the surface of the substrate is kept at 20xc2x0 or less (preferably, 10xc2x0 or less) at least immediately before pre-washing and drying the substrate prior to deposition of a film on the substrate after it is unpacked With the structure 1-5, the contact angle of water on the surface of the substrate is kept at 20xc2x0 or less (preferably, 10xc2x0 or less) for the period immediately after the washing/drying step and immediately before a pre-washing and drying step performed before the film is deposited on the substrate which is unpacked. Thus, even if spin dry is performed after the substrate is washed by DI water alone, the protrusions containing at least one of C, O, Al, Si, Fe, Cu, Zn, and Zr and having a height causing the product defects are not formed. Therefore, it is possible to shorten the production process and to lower the cost.
For example, the contact angle of water on the surface of the substrate is kept at 20xc2x0 or less (preferably, 10xc2x0 or less) during the above-mentioned period by (1) shortening a time period required to wash, inspect, and package the substrate, (2) using, as a case or a bag for packaging the substrate, a packaging material which produces a less amount of a release gas or a less number of particles, and (3) packaging the substrate in the case or the bag with a desiccant contained therein.
The structure 1-6 is defined because this invention is particularly effective in case where the substrate is the glass substrate.
The structure 1-7 is defined because the above-mentioned protrusions cause a serious problem particularly in the substrate for the magnetic disk intended to reduce the flying height of a magnetic head.
The substrate for an information recording medium according to this invention is adapted to the substrate for the magnetic disk subjected to recording/reproducing operations with an extremely small distance kept between a slider and the surface of the magnetic disk.
Furthermore, the above-mentioned protrusions cause a serious problem particularly in the substrate for the magnetic disk to be used with the magnetoresistive head (MR head) intended to prevent the thermal asperity, more particularly in the substrate for the magnetic disk to be used with the giant magnetoresistive head (GMR head). Therefore, the above-mentioned substrate for a magnetic disk is particularly effective for use with the magnetoresistive head or the giant magnetoresistive head.
In the structure 1-8, in the method of producing an information recording medium, the step of washing ad drying the substrate for the information recording medium prior to deposition of the film on the substrate (particularly, the step of pre-washing and drying the substrate immediately before deposition) has the content similar to that of the structure 1-1. This is because the spin dry is carried out in the step of washing and drying the substrate for the information recording medium prior to deposition of the film on the substrate and because, even if the above-mentioned protrusions are not formed on the substrate, product defects can not be avoided in case where the protrusions are formed during the step of washing and drying the substrate prior to deposition.
The structure 1-9 is effective as mentioned in conjunction with the structure 1-2.
With the structure 1-10, the contact angle of water on the surface of the substrate before washing prior to deposition (particularly, pre-washing immediately before deposition) of a film on the substrate is equal to 20xc2x0 or less. Thus, the height of the protrusions formed on the substrate and containing at least one of C, O, Al, Si, Fe, Cu, Zn, and Zr as a main component is reduced to a level such that the product defects are not caused when the information recording medium is made.
For example, the contact angle of water on the surface of the substrate before washing prior to deposition is controlled to be equal to 20xc2x0 or less by the method described in conjunction with the structure 1-4 or by treating the substrate in a washing bath (acid, alkali, neutral detergent, and the like) or a hydrophilic treatment bath (silicofluoric acid) prior to deposition.
This invention is particularly effective in the washing/drying step using a drying technique which does not require a chemical solution. For example, such drying method may be the spin dry, air knife (drying by blowing compressed air), and heat drying. The structure 1-11 is defined because the protrusions are produced particularly in the washing/drying step using the spin dry.
Typically, the spin dry performs dehydration alone by high-speed rotation. The spin dry is applicable to both of cassette type (batch type) processing and sheet-feed type processing. In the spin dry, a clean gas such as nitrogen can be made to flow in a desiccator or can be blown to the surface of the substrate. The spin dry includes a method of spraying a pure water shower onto the surface of the substrate to wash or rinse the substrate and to prevent the substrate from being dried, then stopping the pure water shower, and drying the substrate by high speed rotation, a method of directing a jet of high-pressure water (pure water) onto the surface of the substrate being rotated at a high speed to wash the substrate, then stopping the jet of the high-pressure water, and drying the substrate by high speed rotation, and so on.
The washing/drying step using the spin dry includes the spin dry after the washing by the DI water alone, the spin dry after rinsing with the DI water, or the like.
The structure 1-12 is defined because this invention is effective in case where the substrate is the glass substrate excellent in flatness and smoothness.
The structure 1-13 is defined because the above-mentioned protrusions cause a serious problem particularly in the magnetic disk intended to reduce the flying height of the magnetic head.
This invention is particularly effective in case where the magnetic disk is to be mounted in an information recording apparatus using a magnetoresistive head or a giant magnetoresistive head as a recording/reproducing head.
Structure 2-1
A method of producing a substrate for an information recording medium, the method including a washing/drying step of washing and drying the substrate subjected to precision polishing, the method comprising:
preliminarily calculating a correlation between a content of at least one element of C, Al, Si, Fe, Cu, Zn, and Zr contained in water used in the washing/drying step and a size (height) of protrusions attached to the surface of the substrate after the washing/drying step; and
determining the content of the element contained in the water used in the washing/drying step so that the protrusions will have such a size (height) that no hit is caused when at least a recording layer is formed on the substrate to produce the information recording medium and a slider provided with a recording device and/or a reproducing device is made to run afloat on a surface of the information recording medium and/or that no error is caused upon carrying out a recording operation and/or a reproducing operation.
Structure 2-2
A method of producing a substrate for an information recording medium, the method including a washing/drying step of washing and drying the substrate subjected to precision polishing, wherein:
the content of at least one element of C, Al, Si, Fe, Cu, Zn, and Zr contained in water used in the washing/drying step is equal to 20 ppb or less.
Structure 2-3
A method as described in the structure 2-1 or 2-2, wherein drying in the washing/drying step is spin dry.
Structure 2-4
A method as described in any one of the structures 2-1 through 2-3, wherein the substrate is a substrate for a magnetic disk.
Structure 2-5
A method as described in the structure 2-4, wherein the substrate is a substrate for a magnetic disk to be used with a magnetoresistive head or a giant magnetoresistive head.
Structure 2-6
A method of producing an information recording medium, the method including the steps of preparing a substrate subjected to precision polishing, pre-washing and drying the substrate before depositing a film thereon, and forming at least a recording layer on the substrate after the pre-washing and drying step, the method comprising:
preliminarily calculating a correlation between a content of at least one element of C, Al, Si, Fe, Cu, Zn, and Zr contained in water used in pre-washing in the pre-washing and drying step and a size (height) of protrusions attached to the surface of the substrate after the pre-washing and drying step; and
determining the content of the element contained in the water used in the pre-washing and drying step so that the protrusions will have such a size (height) that no hit is caused when at least a recording layer is formed on the substrate to produce the information recording medium and a slider having a recording device and/or a reproducing device is made to run afloat on a surface of the information recording medium and/or that no error is caused upon carrying out a recording operation and/or a reproducing operation.
Structure 2-7
A method of producing a substrate for an information recording medium, the method including the steps of preparing a substrate subjected to precision polishing, pre-washing and drying the substrate before depositing a film thereon, and forming at least a recording layer on the substrate after the pre-washing and drying step, wherein:
the content of at least one element of C, Al, Si, Fe, Cu, Zn, and Zr contained in water used in the pre-washing and drying step is equal to 20 ppb or less.
Structure 2-8
A method as described in the structure 2-6 or 2-7, wherein drying in the pre-washing and drying step is spin dry.
Structure 2-9
A method as described in any one of the structures 2-6 through 2-8, wherein the substrate is a substrate for a magnetic disk.
Structure 2-10
A method as described in the structure 2-9, wherein the substrate is a substrate for a magnetic disk to be used with a magnetoresistive head or a giant magnetoresistive head.
According to the structure 2-1, the content of at least one element of C, Al, Si, Fe, Cu, Zn, and Zr contained in the water used in the washing/drying step and causing protrusions containing at least one of C, O, Al, Si, Fe, Cu, Zn, and Zr is determined and controlled so that product defects can be avoided. Thus, it is possible to avoid product defects.
Specifically, the content of at least one element of C, Al, Si, Fe, Cu, Zn, and Zr contained in the water used in the washing/drying step is determined and controlled so that, when at least the recording layer is formed on the substrate to produce the information recording medium, protrusions will have a size (height) too small to hit the slider provided with the recording and/or the reproducing device and/or that no error is caused upon carrying out the recording operation and/or the reproducing operation. Therefore, it is possible to prevent occurrence of head crash and thermal asperity and to eliminate negative factors causing a defect such as an error in the recording operation and/or the reproducing operation of the information recording medium. The height of the protrusions is represented by Rp or the like and is preferably equal to 0 nm (i.e., no protrusion is formed). The height of the protrusions may be a value obtained by directly measuring only the protrusions or may be a value obtained by measuring the surface roughness of the substrate before and after the washing/drying step.
In this invention, the washing/drying step involves a washing operation immediately after precision polishing (final polishing), a washing operation after chemical reinforcement, a washing operation before shipment, a washing operation immediately before deposition, or any other washing operation carried out after the precision polishing and before the deposition. Once the protrusions including at least one of C, O, Al, Si, Fe, Cu, Zn and Zr are formed in any one of the washing/drying steps corresponding to the above-mentioned washing operations, such protrusions may not be removed by the subsequent various washing steps. Therefore, it is preferable that the content of at least one element of C, Al, Si, Fe, Cu, Zn, and Zr contained in the water used in each of these washing/drying steps is determined and controlled so as to avoid product defects.
The washing/drying step generally comprises (1) a washing step using acid, alkali, a neutral detergent, and any other appropriate cleaning agent, (2) a rinsing step using pure water or a solvent, and (3) a drying step. However, the washing/drying step may be a simple process comprising dipping into DI water followed by spin dry.
In the structure 2-2, the content of at least one element of C, Al, Si, Fe, Cu, Zn, and Zr contained in the water used in the washing/drying step of the substrate for an information recording medium is equal to 20 ppb or less so that the size (height) of the protrusions formed on the substrate and containing at least one element of C, O, Al, Si, Fe, Cu, Zn, and Zr is reduced so as to avoid the product defects when the information recording medium is produced.
As the content of at least one element of C, Al, Si, Fe, Cu, Zn, and Zr contained in the water used in the washing/drying step of the substrate is smaller, the size (height) of the protrusions containing at least one of C, O, Al, Si, Fe, Cu, Zn, and Zr is reduced (lowered). Specifically, the content is preferably equal to 15 ppb or less, more preferably, 10 ppb or less.
The content of the element or elements contained in the water can be obtained, for example, by the absorptionmetric analysis or the ICP (Inductively Coupled Plasma) atomic emission spectroscopy. The content of the above-mentioned element contained in the water can be appropriately adjusted, for example, by repetition of filtering or ion exchange. The height of the protrusions is represented by Rp or the like and is preferably equal to 0 nm (i.e., no protrusion is formed).
The protrusions are formed particularly when a drying technique without using a chemical solution is performed. For example, the drying technique without using the chemical solution may be the spin dry, air knife (drying by blowing compressed air), and heat drying, The structure 2-3 is defined because the protrusions are formed particularly in the washing/drying step using the spin dry.
The structure 2-4 is defined because the above-mentioned protrusions cause a serious problem particularly in the substrate for the magnetic disk intended to reduce the flying height of a magnetic head.
The substrate for an information recording medium according to this invention is adapted to the substrate for the magnetic disk subjected to recording/reproducing operations with an extremely small distance kept between a slider and the surface of the magnetic disk.
The structure 2-5 is defined because the above-mentioned protrusions cause a serious problem particularly in the substrate for the magnetic disk to be used with the magnetoresistive head (MR head) intended to prevent the thermal asperity, more particularly in the substrate for the magnetic disk to be used with the giant magnetoresistive head (GMR head).
The structure 2-6 defines the method of producing an information recording medium, in which the content similar to that of the structure 2-1 is applied to the step of washing and drying the substrate for the information recording medium prior to deposition of the film on the substrate (particularly, the step of pre-washing and drying the substrate immediately before deposition). This is because the spin dry is often carried out in the step of washing and drying the substrate for the information recording medium prior to deposition of the film on the substrate and because, even if the above-mentioned protrusions are not formed on the substrate, product defects can not be avoided in case where the protrusions are formed during the step of washing and drying the substrate prior to deposition.
With the structure 2-7, the content of at least one element of C, Al, Si, Fe, Cu, Zn, and Zr contained in the water used in the step of washing (particularly, pre-washing immediately before deposition) and drying the substrate prior to deposition of a film on the substrate is controlled to be equal to 20 ppb or less. Thus, the size (height) of the protrusions formed on the substrate and containing at least one of C, O, Al, Si, Fe, Cu, Zn, and Zr is reduced to a level such that the product defects are not caused when the information recording medium is made.
As the content of the above-mentioned element contained in the water used in the pre-washing and drying step is smaller, the size (height) of the protrusions containing at least one of C, O, Al, Si, Fe, CU, Zn, and Zr is reduced (lowered). Specifically, the content is preferably equal to 15 ppb or less, more preferably, 10 ppb or less. The height of the protrusions is represented by Rp or the like and is preferably equal to 0 nm (i.e., no protrusion is formed).
The protrusions are formed particularly when a drying technique without using a chemical solution is performed. For example, the drying technique without using the chemical solution may be the spin dry, air knife (drying by blowing compressed air), and heat drying. The structure 2-8 is defined because the protrusions are formed particularly in the washing/drying step using the spin dry.
Typically, the spin dry performs dehydration alone by high-speed rotation. The spin dry is applicable to both of cassette type (batch type) processing and sheet-feed type processing. In the spin dry, a clean gas such as a nitrogen gas can be made to flow in a desiccator or can be blown to the surface of the substrate. The spin dry includes a method of spraying a pure water shower onto the surface of the substrate to wash or rinse the substrate and to prevent the substrate from being dried, then stopping the pure water shower, and drying the substrate by high speed rotation, a method of directing a jet of high-pressure water (pure water) onto the surface of the substrate being rotated at a high speed to wash the substrate, then stopping the jet of the high-pressure water, and drying the substrate by high speed rotation, and so on.
The washing/drying step using the spin dry includes the spin dry after the washing by the DI water alone, the spin dry after rinsing with the DI water, or the like.
The structure 2-9 is defined because the above-mentioned protrusions cause a serious problem particularly in the magnetic disk intended to reduce the flying height of the magnetic head.
With the structure 2-9, the magnetic disk capable of carrying out high-density recording/reproducing operations can be obtained.
The structure 2-10 is defined because the above-mentioned protrusions cause a serious problem particularly in the substrate for the magnetic disk to be used with the magnetoresistive head (MR head) intended to prevent the thermal asperity, more particularly in the substrate for the magnetic disk to be used with the giant magnetoresistive head (GMR head).
With the structure 2-10, the magnetic disk capable of carrying out higher-density recording/reproducing operations can be obtained.
The height of the protrusions containing at least one of C, O, Al, Si, Fe, Cu, Zn, and Zr depends upon the surface condition (wettability) of the surface of the substrate. If the wettability of the surface of the substrate is improved (i.e., the contact angle of water becomes small), water droplets of the cleaning water spread over the surface of the substrate and are dried. In this event, C, O, Al, Si, Fe, Cu, Zn, and/or Zr forming the protrusions do not concentrate to particular regions but are dispersed throughout the surface of the substrate.
As described above, if the content of at least one element of C, Al, Si, Fe, Cu, Zn, and Zr contained in the water used in the washing/drying step is equal to 20 ppb or less, the protrusions containing at least one of C, O, Al, Si, Fe, Cu, Zn, and Zr and having the size (height) causing the product defects are not formed in general. Furthermore, it the content of at least one element of C, Al, Si, Fe, Cu, Zn, and Zr contained in the water is controlled to 20 ppb or less and simultaneously if the wettability of the surface of the substrate is improved, the protrusions are more hardly produced.
The contact angle of water on the surface of the substrate is preferably equal to 20xc2x0 or less, more preferably, equal to 10xc2x0 or less. The contact angle of water is measured by the sessile drop method.
The contact angle of water (wettability) depends upon the amount of organic substances attached to the surface of the substrate, the surface roughness of the substrate, the material of the substrate, and the like. Among others, the amount of the organic substances attached to the surface of the substrate greatly affects the contact angle. Therefore, it is desirable to store the substrates under an environment capable of keeping the contact angle at a small value, i.e., an environment in which a less amount of the organic substances are produced from a substrate case or a bag and attached to the surface of the substrate.
Structure 3-1
A substrate for an information recording medium, wherein protrusions attached and formed on a surface of the substrate and containing at least one of C, O, Al, Si, Fe, Cu, Zn, and Zr have a height (Rp) such that no hit is caused when at least a recording layer is formed on the substrate to produce the information recording medium and a slider provided with a recording device and/or a reproducing device is made to run afloat on a surface of the information recording medium and/or that no error is caused upon carrying out a recording operation and/or a reproducing operation.
Structure 3-2
A substrate as described in the structure 3-1, wherein the height (Rp) of the protrusions is such that no hit is caused when the slider is made to run afloat with respect to the surface of the information recording medium at a flying height of 30 nm or less and/or that no error is caused upon carrying out a recording operation and/or a reproducing operation.
Structure 3-3
A substrate as described in the structure 3-1 or 3-2, wherein the height of the protrusions attached and formed on the surface of the substrate falls between 0 and 30 nm in terms of Rp.
Structure 3-4
A substrate as described in any one of the structures 3-1 through 3-3, wherein the protrusions have a ratio Rp/Rv which is not smaller than 10.
Structure 3-5
A substrate as described in any one of the structures 3-1 through 3-4, wherein an interface indicative of attachment of foreign matters is present between the protrusions and the surface of the substrate.
Structure 3-
A substrate as described in any one of the structures 3-1 through 3-5, wherein the protrusions are formed by an impurity element contained in water used in a washing/drying step of washing and drying the substrate.
Structure 3-7
A substrate as described in the structure 3-6, wherein a contact angle of water on the surface of the substrate before the washing/drying step is equal to 20xc2x0 or less.
Structure 3-8
A substrate as described in any one of the structures 3-1 through 3-7, wherein the substrate is a substrate for a magnetic disk.
Structure 3-9
An information recording medium comprising a substrate described in any one of the structures 3-1 through 3-8 and at least a recording layer formed thereon.
Structure 3-10
An information recording medium as described in the structure 3-9, wherein the recording layer is a magnetic layer.
Structure 3-11
An information recording medium as described in the structure 3-10, wherein the information recording medium is a magnetic disk for use with a magnetoresistive head or a giant magnetoresistive head.
According to the structure 3-1, the protrusion attached and formed on the surface of the substrate and containing at least one of C, O, Al, Si, Fe, Cu, Zn, and Zr has a height (Rp) such that the product defects can be avoided. Thus, the product defects can be avoided.
Specifically, the protrusion attached and formed on the surface of the substrate and containing at least one of C, O, Al, Si, Fe, Cu, Zn, and Zr has a height (Rp) such that no hit is caused when at least a recording layer is formed on the substrate to produce the information recording medium and a slider provided with a recording device and/or a reproducing device is made to run afloat on a surface of the information recording medium and/or that no error is caused upon carrying out a recording operation and/or a reproducing operation. Thus, it is possible to prevent occurrence of head crash and thermal asperity and to eliminate negative factors causing a defeat such as an error in the recording operation and/or the reproducing operation of the information recording medium. Herein, the height of the protrusions may be a value obtained by directly measuring only the protrusions or may be a value obtained by measuring the surface roughness of the substrate before and after the washing/drying step.
As described in the structure 3-2, the slider has a flying height which is preferably equal to 30 nm or less, more preferably 20 nm or less, in order to enable recording and reproducing operations at a high density. Depending upon a recording/reproducing system, the slider may be brought into pseudo contact with the surface of the recording medium or the slider may run in contact therewith. In such events, the flying height of the slider may be equal to 0 nm. Therefore, this invention includes the case where the flying height of the slider is equal to 0 nm.
In the structure 3-3, the height of the protrusions attached and formed on the surface of the substrate is controlled to fall between 0 and 30 nm in terms of Rp. With this structure, it is possible to avoid the hit of the slider when the slider has a flying height of 30 nm. Furthermore, in case where the slider has a flying height smaller than 30 nm and 20 nm, the height of the protrusions are controlled to fall between 0 and 20 nm and between 0 and 10 nm in terms of Rp, respectively. Thus, it is possible to obtain the information recording medium capable of carrying out the recording and the reproducing operations at a high density without causing a hit of the slider and a recording or a reproducing error.
Herein, Rp represents a distance from a mean line to a highest peak. Rp can be measured by the atomic force microscope or a probe-type surface roughness meter such as Talystep (Taylor-Hobson). In this invention, it is necessary to obtain accurate and strict information about the surface condition of the substrate (medium) in order to achieve high-density recording and reproducing operations. Therefore, the value measured by the atomic force microscope is used as Rp.
It is noted here that the value of Rp never exceeds the maximum roughness Rmax. Rmax has a correlation with the flying height of the slider afloat on the surface of the medium. Unless Rmax is sufficiently smaller than the flying height of the slider, the head inevitably collides with the protrusions. In case of a hard disk of the CSS (Contact Start Stop) system in which the flying height of the slider is 30 nm or less, it is practically desired that Rmax is equal to about 10 nm. In case of a hard disk of the CSS system in which the flying height of the slider is 20 nm or less, Rmax is required to have a further small value. In case where the recording or the reproducing operation is carried out while the slider is kept In contact with the surface of the medium, it is necessary to keep the surface condition such that the slider is not attracted and attached to the surface of the medium.
In the structure 3-4, Rv represents the height from the center line to the bottom. Rv is produced by polishing and has a value between 2 and 3 nm irrespective of presence or absence of the protrusions. If the surface roughness is small, Rv has a smaller value. The surface of the substrate obtained by typical grinding and polishing has a ratio Rp/Rv smaller than 10. On the other hand, the protrusions causing the product defects are indicated by the ratio Rp/Rv not smaller than 10 and are produced due to the presence of impurity elements contained in the cleaning water. In the substrate for the magnetic disk to be used with the slider having a flying height of 30 nm or less, the protrusions causing the product defects have a height not smaller than 20 to 30 nm. Therefore, if the ratio Rp/Rv of the surface of the substrate is great, this means that the protrusions causing the product defects are formed. If the ratio Rp/Rv of the surface of the substrate is smaller than 10, the protrusions are very low in height and do not cause the product defects even if the protrusions are formed.
As described in the structure 3-5, the protrusions can be judged as the foreign matters attached to the substrate in case where the interface is present between the protrusions and the surface of the substrate.
Thus, since the protrusions are substances attached to the substrate, the protrusions are similarly produced in any substrate other than the glass substrate.
As described in the structure 3-6, the protrusions are formed by the impurity elements (C, O, Al, Si, Fe, Cu, Zn, or Zr) contained in the water used in the washing/drying step. From the experimental results which will later be described, it has been confirmed that the protrusions depend upon the surface condition (wettability) of the surface of the substrate. If the wettability of the surface of the substrate is improved (i.e., the contact angle of water becomes small), water droplets of the cleaning water containing the impurity elements spread over the surface of the substrate and are dried. In this event, the impurity elements forming the protrusions do not concentrate to particular regions but are dispersed throughout the surface of the substrate so that the protrusions are not formed or, even formed, are extremely small in size. On the contrary, if the contact angle of water on the surface of the substrate is increased, each water droplet of the cleaning water containing the impurity elements concentrates to a single spot and is then dried. Therefore, the above-mentioned problems are caused and the protrusions are formed.
The protrusions are formed particularly when a drying technique without using a chemical solution is performed. For example, the drying technique without using the chemical solution may be the spin dry, air knife (drying by blowing compressed air), and heat drying. In these drying techniques, a small amount of the DI water is left on the surface as a thin film so that the protrusions are readily formed, unlike the washing/drying technique using IPA.
Accordingly, as described in the structure 3-7, it is preferable that the contact angle of water on the surface of the substrate is reduced in case where the substrate is washed and dried as described in the structure 3-6. Specifically, the contact angle is preferably equal to 20xc2x0 or less, more preferably, 10xc2x0 or less. The contact angle of water is measured by the sessile drop method.
The structure 3-8 is defined because the above-mentioned protrusions cause a serious problem particularly in the substrate for the magnetic disk intended to reduce the flying height of a magnetic head.
The substrate for an information recording medium according to this invention is adapted to the substrate for the magnetic disk subjected to recording/reproducing operations with an extremely small distance kept between the slider and the surface of the magnetic disk.
As described in the structure 3-9, the magnetic disk capable of carrying out high-density recording and reproducing operations can be obtained by forming the magnetic layer on the substrate described in any one of the structures 3-1 and 3-8.
As described in the structure 3-10, the information recording medium suppressed in occurrence of defects can be obtained by forming a recording layer on the substrate described in any one of the structures 3-1 through 3-8.
The structure 3-11 is defined because the above-mentioned protrusions cause a serious problem particularly in the magnetic disk to be used with the magnetoresistive head (MR head), more particularly in the magnetic disk to be used with the giant magnetoresistive head (GMR head).
With the structure 3-11, the magnetic disk capable of carrying out higher-density recording/reproducing operations can be obtained.